I'm going to step back and try a different angle, because a certain fraction of my numbers are indefensible, as you pointed out above.

In the post above that started all this, I made two basic claims.

1. The inverted population pyramid (a mountain of elderly supported on a tiny point of young, productive workers) was unlikely to happen because of immigration.

2. Even if we did end up with an inverted population pyramid, it probably wouldn't matter.

To support #2, I relied upon the following assumption, which was hidden in my numbers:

2a. The US GDP will continue to grow, even with a reduced workforce.

#2a is really the taken-for-faith axiom at the heart of my argument. If the US GDP increases faster than population, then the per-capita income will always increase as well, even if the employment-population ratio plummets. If the per-capita income increases, then the standard of living increases for everyone, employed or unemployed. In the end, that is what separates a utopian future from a dystopian future. All the rest of my numbers were just me playing with the math to see how the redistribution might look. They don't do anything to prove #2a.

Now, since it's bad form to abandon an argument in progress, I'll try to defend my math a little, or at least extend it where it is lacking.

I am seeing three basic arguments in your post:
1. A claim that I am double-counting sales tax.

2. A rejection of the way I just lumped business taxes in with personal income taxes.

3. A claim that I am not counting non-federal taxes in the 2050 tax rate.

First, all of my numbers are coming from http://www.usgovernmentrevenue.com/, for 2012. I have two additional numbers to enter into record, $13 trillion (total US personal income) and $15 trillion (US GDP).

Regarding sales tax, I merely phrased things poorly. The first mention, I was using sales tax as an example of something which is not considered an income tax but most certainly comes out of person income. The second mention, I was using sales tax as an example of something not levied at a federal level. My $5.1 trillion number is someone else's total, so I at least did not double count anything going into it.

Regarding personal versus business taxes, this is a very legitimate criticism. The best argument I could make is that the burden of business taxes ultimately fall on individuals, either the customers or on the owners and employees. This isn't quite sufficient: while business taxes which fall upon customers are ultimately paid out of their already reported personal income, but the business taxes which fall upon owners and employees come out of money which is never reported as personal income. This would make the tax rate $5.1 trillion / ($13 trillion + $X), where $X is the subset of taxes which are paid by businesses.

I am going to make the claim that $13 trillion + $X is roughly $15 trillion, the US GDP. From one angle, it makes sense. The US GDP is $15 trillion, the US gov't takes $2 trillion off the top in corporate income taxes, property taxes paid by business, the employer contribution to payroll taxes, and other business taxes, and the other $13 trillion ends up as personal income. From another angle, eyeballing the breakdown of taxes (income, payroll, ad-valorem, fees, business) it looks like something like $1.5-$2.5 trillion are paid by business instead of individuals.

So taking the tax base to be the GDP instead of the total personal income, taxes right now are 33% (5.1 / 15) of the US GDP. Taxes paid by individuals are 23.8% (3.1 / 13). Average post-tax income is $53,000 (76% * $70,000).

In the future I described, the US GDP (2% growth) is $31.8 trillion. Total US tax revenue is still $19 trillion. Taxes are then 60% of the US GDP. Business taxes are $4.8 trillion (US GDP - payroll, 31.8 - 27), so personal taxes are $14.2 trillion dollars. Taxes paid by individuals in 2050 are 52.6%. Average post-tax income is $128,000 in 2050 (47.4% * $270,000).

So shifting taxes from individuals to businesses makes the future even brighter than previously described: personal post-tax income is now more than double 2012, instead of nearly double.

Regarding state and local taxes in 2050, they are included in the $19 trillion figure. I started with the $5.1 trillion 2012 figure and grew from there. Since the 2012 figure included state and local taxes, the 2050 figure does as well.